This invention relates to an underinflation detector that detects insufficient inflation pressure of tires of a vehicle.
In order to achieve stable driving of a vehicle, a variety of systems for detecting loss or insufficiency of inflation pressure of tires have been developed. Typically, these systems fall roughly into two approaches: one that detects inflation pressure directly by a pressure sensor attached to each wheel; and the other that determines change in inflation pressure from a difference in rotational speeds of tires by making use of outside diameters of the tires which changes in accordance with change in the inflation pressure. The latter approach that determines change in inflation pressure from differences in rotational speeds of the tires has an advantage that a rotational speed sensor attached to each wheel for the antilock brake system or the like in ordinary vehicles can be used to construct the system.
Among techniques for detecting insufficiency of inflation pressure through the latter approach in which difference in rotational speeds of the tires is utilized is, for example, a method adopted by an apparatus disclosed in JP 49-85701 A in which the sum of the numbers of revolutions per unit time of the left front wheel and the right rear wheel is calculated, the sum of the numbers of revolutions per unit time of the right front wheel and the left rear wheel is calculated, and the sums of the numbers of revolutions per unit time as thus calculated are compared. This apparatus can accordingly achieve improved accuracy of detection by offsetting the difference in the numbers of revolutions per unit time between the right wheel and the left wheel which arises while a vehicle makes a turn in a curved road, and the difference in the numbers of revolutions per unit time between the front wheel and the rear wheel which is derived from the difference between the driving wheel and the wheel driven by the driving wheel.
Another technique for detecting insufficiency of inflation pressure through the approach in which difference in rotational speeds of the tires is utilized is disclosed in JP 6-92114 A, in which a control unit is programmed so as not to detect the difference in the rotational speeds of the tires in transient states of behavior of the vehicle, for example, where the vehicle drives at a very low speed, or the speed of the vehicle increases or decreases suddenly, or the vehicle undergoes a great lateral acceleration. Thus-configured apparatus can accordingly prevent a possible error in detection, thereby increasing reliability.
However, when tire inflation pressure of one of front wheels decreases, for example, during constant-speed straight-ahead driving of a vehicle, a rotational speed of the wheel increases in accordance with the reduction of an outside diameter thereof. On, the other hand, the contact area of the wheel with the road increases accordingly; thus, the rolling resistance thereof increases as well. When the rolling resistances of two front wheels connected by one and the same axle shaft are different from each other, a difference occurs between forces acting on each front wheel, with the result that a very small yaw rate is observed in the front wheels. It is likely, in such an instance, that a driver would steer so as to cancel the yaw rate irrespective of whether or not the driver intends to do so (steering for correction). Thus, a yaw rate in a direction opposite thereto so as to cancel the front wheel yaw rate would be observed in the rear wheels. Consequently, a difference in rotational speed which results from cornering of the vehicle arises between the rear wheels of which tires have not been underinflated, and thus a disparity between the front-wheel rotational speed difference and the rear-wheel rotational speed difference is reduced apparently on calculation. The conventional techniques for detecting insufficiency of inflation pressure have given no consideration to such influence of steering for correction made almost unconsciously by the driver as described above, and could thus fail to detect the insufficiency of inflation pressure in some instances as above. Therefore, the present invention has been made to address the above disadvantages, and it is an exemplary general object of the present invention to provide an underinflation detector that can detect insufficient inflation pressure of tires with improved reliability.